In motor vehicle transmissions one distinguishes between so-called power-shift transmissions on the one hand side being transmissions having an uninterrupted flow of torque during gearshift and transmissions having an interrupted flow of torque during gearshift on the other hand side. Power-shift transmissions are mostly configured as planetary transmissions having a hydrodynamic torque converter at their input. By actuating clutches and brakes for the various elements within the planetary gear trains the various gears of such a power-shift transmission are actuated in an overlapping relationship so that during a gear change no interruption of torque occurs between the driving engine and the vehicle wheels.
In transmissions having an interrupted flow of torque, for example in conventional stepped motor vehicle transmissions of the countershaft type, the flow of torque is first interrupted during each gearshift when the old gear is separated and is then again closed when the new gear is engaged so that an interruption of the driving force occurs during such a gearshift. This interruption of driving force results in a so-called gearshift jolt which is disturbing is particular during upshift from a low gear to a higher gear because in such a situation the vehicle effects a pitch movement as a consequence of the drive force interruption.
In the course of the automatization of stickshift transmissions with interrupted drive force one has attempted to reduce the gearshift jolt by using two concentrically arranged and structurally integrated friction clutches for overlappingly connecting a first and a second group of gears of the transmission, respectively, with the output of the drive engine. Such transmissions are referred to in the art as "double clutch transmissions" or "twin clutch transmissions". An example of a double clutch transmission of the afore-mentioned kind is discussed in German disclosure document 38 12 359.
Other double clutch transmissions of similar type are described in German disclosure document DE 44 01 812, DE 40 31 851 or in German patent specifications DE 195 48 622 and DE 31 31 139.
In these prior art transmissions the double clutch is configured as a component having a coaxial arrangement of the two clutches in one common clutch housing, however, the function is slightly different as compared to the basic principle of double clutch transmissions discussed further above. In these transmissions one of the friction clutches of the double clutch is used for engaging the respective highest gear, for example the sixth gear (DE 44 01 812) during each gearshift. As a consequence a certain torque-transmitting connection remains via the highest gear during all other gearshift operations in contrast to conventional stepped transmissions with drive force interruption where the flow of torque is entirely interrupted between the engine and the transmission output during gearshift.
In this type of transmissions the gearshift jolt is slightly reduced, however, the limited gain in comfort that may be achieved with such transmissions does not justify the substantial structural complications within the transmission.
Furthermore, transmissions of this type have the systematic disadvantage that only the smallest available torque, namely the torque associated to the highest gear, may be coupled to the output. During upshift from first to second gear, where the greatest losses in comfort occur due to gearshift jolts, only minimal gains in comfort may be achieved because the sudden reduction in drive force in first gear may only be reduced to an extremely small extent. Only during upshift in still higher gears the continuing involvement of the highest gear during gearshift becomes more and more effective, however, in such upshift operations in higher gears the gearshift jolt problem is of much lesser importance.
Moreover, these prior transmissions have the disadvantage that for effecting the desired torque bypass via the highest gear the clutch for the highest gear must operate in slip-operation during the entire gearshift. When the vehicle is used in a mode of operation with numerous gearshifts, for example when it is used within urban traffic, this results in substantial wear of the friction clutch and, accordingly, to a significantly reduced clutch lifetime. This holds true in particular because the differential revolution in the slip-operated clutch of the highest gear is the higher, the lower the involved lower gear is and becomes small only when upshifting into higher gears. In urban traffic, however, one mostly shifts between lower gears, for example when starting at an intersection or at a traffic light so that driving in urban traffic has a negative impact on the wear and the lifetime of the highest gear clutch.
German patent specification 38 12 327 describes a double clutch transmission where the first of the two clutches engages the first gear and the other clutch engages the second gear. In this prior art transmission both clutches may be closed simultaneously and the vehicle may be started with both the first and the second gear engaged. As soon as the revolutions in the two clutches are equal, the flow of force through the first gear clutch is interrupted, for example by means of a freewheel. By doing so, the power dissipated during extreme starting operations, for example on steeply ascending roads may be distributed on both clutches.
It is, therefore, an object underlying the invention to improve a stepped transmission of the kind mentioned at the outset such that a substantial improvement in comfort is achieved in the lowest gears which are of particular importance with respect to the gearshift jolt phenomenon. It is, further, an object of the invention to obtain such a transmission with the littlemost engineering efforts and without any losses in transmission lifetime.